The slaughter stone lies outside the main henge, to the left of the entrance, and appears to be streaked with the blood of various sacrificial victims from these days of gore.

However...
There is no evidence that the stone was used as a sacrificial tablet. For one thing it would originally have been an upright stone, rather than lying on its side as it is today. Also the 'blood stains' are a natural phenomenon caused by iron ore within the natural matrix of the sarsen rock. There are no human sacrifices anywhere around Stonehenge, which has been a site of intensive archaeological investigation for the past century. At least, there are no human remains which have yet come to light which are connected to the site and ritual practises. The only burial in the area is that of an archer, pierced quite convincingly by a large amount of arrows, that lies within the henge bank.

The Slaughter Stone is a misnomer, a name associated with the mythos of a site which has plagued academics as to its origins for hundreds of years. Despite rigorous excavation Stonehenge, like most other prehistoric sites, is keeping the secrets of its past to itself. With only a paucity of environmental evidence to draw conclusions from, the true nature of these sites will continue to be a mystery open to wild and spurious conjecture, until someone finally invents the time machine. Just hope I'm around to see it!